1. The Field of the Invention
This invention relates to integrated circuit structures. More particularly, the present invention relates to resistive structures and methods for forming same on integrated circuits.
2. The Prior Art
One of the most common elements in electrical circuit design is a resistor. Nearly all circuit designs require the inclusion of one or more resistive elements. In the case of discrete electrical components, the resistor is generally the easiest and least expensive component to manufacture when compared to capacitors, inductors, and active components. In the case of integrated circuit elements, however, resistors are often difficult to manufacture. In particular, resistors having high resistive values are difficult to incorporate as part of an integrated circuit.
In general, the resistance value exhibited by a resistor is determined by the cross sectional area of the resistive material and the length of the resistive material. As the length of the resistive material increases, and the resistivity and the cross sectional area of the resistive material remain constant, the total resistance will increase.
In the case of integrated circuits, the amount of planar surface area available for forming resistive elements is limited. For example, as the size of circuit elements decreases, and the density at which those circuit elements are packed onto each die, the area which can be devoted to resistive elements decreases.
In particular, in the case of Static Random Access Memory (SRAM) integrated circuits it is necessary to include load resistors or resistive elements (generally two) in each memory cell. As integrated circuit technology has progressed, the four or six transistors comprising each SRAM cell have greatly decreased in size.
Nevertheless, even though the transistors in the SRAM cell have decreased in size, the resistive value of the load resistive elements must not decrease, and preferably should increase. It will be appreciated that as the number of cells on each integrated circuit increases, it is desirable to increase the resistance of each load resistive element to keep the total current consumed by the integrated circuit from increasing.
In order to increase the resistance of the structures functioning as load resistive elements, one approach in the art has been to reduce the cross sectional area, i.e., the thickness, of the material, generally polysilicon, forming the resistive structure. As the thickness of the resistive structure is decreased, a resulting increase in the total resistive value of the structure occurs.
Disadvantageously, as the thickness of resistive structure decreases, the occurrence of defects introduced during the manufacturing process and other failures increases dramatically. Thus, as the thickness of the structure decreases to increase its total resistive value to desirable levels, the number of failures increases to unacceptable levels. As the size of other integrated circuit components continues to decrease, the problems encountered with increasing the resistive value possessed by the resistive elements included on integrated circuits will continue to be a problem in the art.
In view of the foregoing, it would be an advance in the art to provide an integrated circuit resistive structure which provides a high resistance value in a small planar area and which does not require additional processing steps. It would also be an advance in the art to provide an integrated circuit resistive structure which can provide increased resistance values while decreasing the planar surface area required by the structure and which can be reliably manufactured and operated.